Vestigial sideband transmission system



Jan. 11, 1966 D. CRITCHLOW ETAL 3,229,209

VESTIGIAL SIDEBAND TRANSMISSION SYSTEM 2 Sheets-Sheet 1 Filed Dec. 18,1962 FIG.1

FIG.2

FREQUENCIES I DATA MEANS FOR PROVIDING A PAIR OF COMPONENT DELAY WORKFILT

LOW-

FILTER CLIPP NARROW CIRCUIT IVID I -LII IITER*- NET- CIRCUIT SQUARE OE TDEMOD-- PASS I5 SYMME 'TRY LAW -ING BAND CIRCUIT T- AIIPL VSEI FILTERMODULA -TOR CARRIER LOW PASS FILTER COMP CE FREOS L I SOUR I I L FIG.4

FIG.3

INVENTORS DALE L. CRITCHLOW ROBERT H. DENNARD ATTORNEY Jan. 11, 1966 D.L. cRlTcHLow ETAL 3,229,209

VESTIGIAL SIDEBAND TRANSMISSION SYSTEM Filed Dec. 18, 1962 2Sheets-Sheet 2 FIG.IO

sum? Low WA l me PASS SOURCEMJ 25 cm%un FILTEQ r f 5 l 21 24 OR J i (20I A 1 SiGNAL I l I l H6. 5 l 2 1 F l G. 8 FIG. 9

g 1 m m DATA I SUMH'L LOW 32 F I' SOURCE PASS l -4 I ClffiHIIT FILTER II 31 I E t i SINGLE L I we United States Patent 3322209 VESTIGIALSIDEBAND TRANSMESSIUN SYSTEM Dale L. (Iritchlow, Lincolndale, and RobertH. Dennard,

Croton-on-Hudson, N.Y., assignors to Iuternationai Busness MachinesCorporation, New York, N.Y., a

corporation of New York Filed Dec. 18, 1962, Ser. No. 245,500 Claims.(Cl. 325-136) The present invention relates to data transmission andreception systems and more particularly to carrier retrival circuits forsynchronous detection in vestigial sideband suppressed carriermodulation transmission.

A data transmission mode that is very favorable in terms of error ratefor a given signal-to-noise ratio is the binary, double sidebandsuppressed carrier modulation system with synchronous detection at thereceiver. The double sideband system using binary coding however,requires a relatively large bandwidth. An acceptable method ofdecreasing the bandwidth requirement is to eliminate one of the twosidebands since each sideband carries all the information. In practice,a small portion or vestige of the suppressed sideband is left in thesignal in order to render the circuitry physically realizable, fromwhich the term vestigial sideband transmission is derived which will behereinafter referred to as VSB transmission is distinct from singlesideband transmission (SSB) in that one complete sideband and a smallportion of the other sideband are suppressed in SSB resulting in abandpass baseband channel.

The use of VSB transmission in combination with synchronous detection atthe receiver is known to be advantageous in that suppressed carriertransmission can be used and quadrature distortion, present whenenvelope detection is used, is eliminated. Synchronous detection,however, requires a method of retrieval of the carrier wave with theproper frequency and phase at the receiver, and heretofore nosatisfactory system has been known for retrieving this carrier wave atthe receiver for use in synchronous detection in a VSB suppressedcarrier modulation system for random binary data with no coderestrictions.

Accordingly, one object of the present invention is to provide animproved VSB transmission system employing synchronous detection.

Another object of the present invention is to provide a VSB transmissionsystem wherein the presence of a pair of component frequencies(including a carrier frequency component) are included in thetransmitted signal.

Another object of the present invention is to provide a VSB transmissionsystem wherein a pair of component frequencies are introduced into thetransmitted signal by the injection of a D.C. signal of proper magnituneinto the data signal.

Still another object of the present invention is to provide a VSBtransmission system wherein a pair of component frequencies areintroduced into the transmitted signal by modulating the data signalwith a digital signal.

A further object of the present invention is to provide a VSBtransmission system wherein a pair of component frequencies are includedin the transmitted signal by a bias distortion of the data signal.

A further object of the present invention is to provide a VSBtransmission system wherein a pair of frequency components added at thetransmitter are utilized for retrieving the carrier at the receiver.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

ice

In the drawings:

FIG. 1 is an illustration of an idealized frequency characteristic of atypical VSB filter.

FIG. 2 is a schematic diagram of a VSB transmission system following theprinciples of the present invention FIG. 3 is a schematic diagram of anembodiment of a circuit in the system of FIG. 2.

FIG. 4 is an illustration of a waveform useful in describing theoperation of the circuit of FIG. 3.

FIG. 5 is a schema-tic diagram of another embodiment of a circuit in thesystem of FIG. 2.

FIGS. 6 and 7 are illustrations of waveforms useful in describing theoperation of the circuit of FIG. 5.

FIG. 8 is a schematic diagram of a further embodiment of a circuit inthe system of FIG. 2.

FIG. 9 is an illustration of a Waveform useful in describing theoperation of the circuit of FIG. 8.

FIG. 10 is an illustration of a filter characteristic of a circuitassociated with the system of FIG. 2.

Referring to FIG. 1, an illustration of an idealized characteristic of atypical VSB filter is shown and is designated by reference character A.The frequency spectrum of the modulator output for a random data signal(hereinafter referred to as the modulation signal) is indicated byreference character B and is seen to include a lower and an uppersideband symmetrical about a carrier frequency f It is graphically seenthat when the modulation signal represented by B is passed through theVSB filter, there will be uniform transmission for all modulation signalfrequency components in the lower sideband which are removed from thecarrier by an amount greater than f and there will be essentially zerotransmission for all modulation signal frequency components in the uppersideband removed from the carrier by an amount greater than f,, wheref5+f is the high est frequency for which the VSB filter has asignificant output and f, is the baseband equivalent of carrier recoverybandwidth. In the frequency range extending from f i to f +f themodulation signal is attenuated according to the slope of the roll-offcharacteristic of the VSB filter which, in the ideal case, is linear.

The frequencies f f and f +f represent the components of a modulationsignal in the upper and lower sidebands corresponding to a data signalhaving a single frequency f (for example a repetitive 1010 binarysignal). It is seen that in VSB transmission, the component of themodulation signal in the upper sideband will not be transmitted to thereceiver. When the VSB signal is sent through a transmission channelhaving frequency translation (such as a telephone line), a singlefrequency component, other than the carrier frequency, is not sufficientto determine the carrier frequency needed for synchronous demodulationat the receiver. Thus synchronous demodulation of the transmitted signalis not readily feasible.

It is true that low frequency components (including D.C.) are present insome data patterns which produce modulation terms in the frequency rangef f to f l-f and permit the carrier to be retrieved. However, some otherrepetitive data patterns do not include a D.C. component or a frequencycomponent lower than f It would be possible to employ only those datapatterns which contain a low frequency component or a D.C. component andavoid using the data patterns which do not. Such code restrictionsunduly limit the transmission system as a whole, particularly sincerepetitive patterns (ie those tending not to have low frequencycomponents) are very common. Attempts have been made to overcome thisproblem and to retrieve the carrier for synchronous de modulation of VSBtransmission in the absence of either a carrier term or a pair ofcomponents of the modulation signal in the transmitted signal. Forexample, one known method is to inject first and second pilot signals atfrequencies within the transmission passband. The pilot signals undergothe same frequency changes and dis tortions as the carrier and thereforemay be employed to retrieve the carrier for synchronous demodulation.The objections to this method is the complexity and that it increasesthe bandwidth and the power level of the transmitted signal and therebyreduces the advantage gained by using VSB transmission.

In the present invention additional circuitry is added to the VSBtransmitter to provide a pair of component frequencies symmetrical aboutthe carrier frequency in the transmitted signal, or the special casethereof which is a component at carrier frequency. The pair of componentfrequencies or the carrier component will then be employed for carrierretrieval at the receiver. In the embodiments to be described, thepresence of a pair of component frequencies or a carrier component inthe transmitted signal is guaranteed by the provision of circuits whichintroduce low frequency components in the data signal, and which insuresthat the low frequency components so provided will not cancel the lowfrequency components which may already be present in some repetitivedata patterns. In the following discussion, the termlow frequencycomponent relates to either a signal which, when introduced into thetransmitter modulator, will produce a pair of component frequencies inthe transmitted signal which are symmetrical about the carrier frequencyin the range f -f to f -i-h, or a DC. component which, when introducedinto the transmitter modulator, will produce a component at carrierfrequency in the transmitted signal.

Both the pair of component frequencies in the range f f tof +f and thecomponent at carrier frequency will hereinafter be referred to as a pairof component frequencies because the component at carrier frequency isactually the special case where the pair of component frequencies areseparated by zero distance and appear at the carrier frequency. Thus,the introduction of low frequency components into the data signalproduce pairs of components in thetransmitted signal symmetrical aboutthe carrier frequency and in the range fi f to f+f1.,

As the frequency of the low frequency components decreases, the distancebetween the pairs of components decreases until the low frequencycomponents become DC. and the pairs of the component frequencies combineat the carrier frequency to produce a carrier component.

Referring to FIG. 2, a VSB transmission system is shown including atransmitter 1 and a receiver 2 electrically connected by a suitabletransmission medium 3 shown, for purposes of example, as a transmissionline. Transmitter 1 includes a source of data signal 4, a lowpass filter6, a balanced modulator 7 which modulates a carrier signal fromoscillator 8 with the data signal, and a VSB filter 9. Balancedmodulator 7 produces an output which is proportional to the product ofthe data signal and the carrier signal and is often referred to in theart as a product modulator. According to the principles of the presentinvention, additional circuitry referred to as means for providing apair of component frequencies in the transmitted signa is included asrep resented by reference numeral 5.

Circuit modifies the data signal from source 4 to provide low frequencycomponents in the data signal which will produce a pair of componentfrequencies in the transmitted signal in the range f f to f +f such thatthe carrier may be retrieved from the transmitted signal at the receiverby circuitry to be later described.

Circuit 5 is not restricted to one particular circuit arrangement forproviding such pair of component frequencies in the transmitted signalbut rather generically represents a variety of circuitry foraccomplishing the sam result.

One example of a method to guarantee the presence of a pair of componentfrequencies in the transmitted signal via the introduction of a lowfrequency component in the data signal is by the injection of a DC.component into the data signal from source 4. More specifically, theinjection of the DC. (i.e. low frequency) component produces a componentat carrier frequency in the transmitted signal. In this method, circuit5 would consist of a DC. supply, such as battery 5A in series withresistor 53, which adds a DC. component to the data signal from source 4at summing circuit 5C as shown in PEG. 3. The DC. signal thus added tothe data signal, when introduced into modulator 7, will result in acomponent at carrier frequency f in the modulation signal from modulator7 which, when transmitted to receiver 2, can be used for synchronousdemodulation. It is necessary to insure that the level of the DC. signalinjected by the battery 5A is such that it will not cancel out D.C.components which may be present in some repetitive data patterns aspreviously mentioned.

Consider that the data signal from data source 4 is the bipolar binarytype of coded signal which varies between :1 unit. An analysis can bemade of the binary signals possible for repetitive data patterns of anylength to determine the possible D.C. components. If such data signalhas produced a pair of frequency components in the range fi -f to f +findependent of the average D.C. component of the signal, it isimmaterial if the injected D.C. signal and the DC. component of the datasignal cancel since the pair of frequency components will provide thenecessary carrier retrieval information in the transmitted signal. Thusit is only necessary to consider data signals having a fundamentalfrequency component greater than f For convenience, the followingdesignations will be employed:

f fundamental frequency of the data signal f =the data rate of the datasignal rz=the pattern length in bits of the data pattern which repeatsIt is known that and since only data patterns wherein f f are beingconsidered, then:

In every system the maximum data rate f wil be known and f will also beknown. Therefore the maximum pattern length n of the data patterns whichwill not have pairs of components in the range f -f to f +f can bedetermined and such data patterns can be analyzed to determine theD.C..levels which may be present. The level of the DC. signal injectedcan then be selected such that cancellation will not occur.

To illustrate, consider a system wherein the values of f and f are suchto produce a ratio of fat/f equal to 5.' Thus the data patterns whichmust be analyzed have pattern lengths (n) of 5 bits or less. The DC.levels of pattern lengths of 5 bits or less can readily be calculated.For example, for the bipolar binary code which varies between :1 unit adata pattern of length one will be at either +1 unit or -1 unit for a i1unit average, a code pattern of length two will consist of a +1 unit anda -1 unit for a zero D.C. average. Note, a pattern consisting of all +1units or all -1 units will not be considered since it is the same as aseries of patterns of length one and will have a DC. level of either +1or 1 unit. A code pattern of length three will consist of either two +1units and one -1, or two 1 units and one +1 unit providing a D.C.average of 1:33 of a unit. A code pattern of length four will consist ofeither two +1 units and two 1 units for a D.C. average of zero, or 1three units and one unit for a D.C. average of ':.50 of a unit. A codepattern of five units will consist of either i three units and two unitsfor a D.C. average of of a unit, or four units and 1 one unit for a D.C.average of $.60 of a unit. Thus, for the system wherein the ratio f /fequals 5, the minimum D.C. level of data signals not producing pairs ofcomponents in the transmitted signal in the range f -f to f -i-f is :.20unit. The amount of D.C. signal injected by battery 5A must be of amagnitude that it will not be cancelled by the aforesaid data signals,therefore, a D.C. signal onehalf the magnitude of the minimum D.C.possible in the aforesaid data signal is selected. In the presentexample the D.C. signal injected by battery 5A would be $0.1 unit. Thusdata patterns previously having a D.C. level of zero will now contain aD.C. component of i0.1 unit and all other data patterns will have atleast a D.C. component of 10.1 unit. In the general case, the ratio f isdetermined, the D.C. levels for data patterns having n less than f f iscalculated, and the DC. level for the injected signal is selected so asnot to cancel with the data signal.

Referring to FIG. 4, the waveform of a typical bipolar binary datasignal which originally varied between plus and minus 1 unit (ofvoltage) is shown after being modified by the addition of 0.1 unit ofD.C. voltage from battery 5. The data signal will now vary between plus1.1 units and minus 0.9 unit insuring at least a 0.1 unit D.C. componentwhich, when introduced into modulator 7, will result in a component atcarrier frequency in the transmitted signal.

The limitation that the magnitude of the D.C. voltage injected bybattery 5A be selected such that no cancellation will occur is necessarywhen the data signal may include data patterns having a D.C. component.This limitation is not necessary when it is known that the data signaldoes not include a DC. component. For example, when particular coding,such as the 4 of 8 code is being employed, it is known that there willbe no D.C. component present and there need be no limitation on themagnitude of the D.C. component injected by battery 5A.

Another embodiment of circuit 5 for providing a pair of componentfrequencies in the transmitted signal by introducing a low frequencycomponent into the data signal is shown in FIG. 5. The circuit of FIG. 5provides means for modulating the data signal by a periodic signalhaving predetermined frequency components such that the modified datasignal contains low frequency components less the f which, whenmultiplied with the carrier signal, results in a pair of componentfrequencies in the transmitted signal in the range 11-13 to f +f Morespecifically, the circuit of FIG. 5 shows an embodiment including asignal generator 20 which produces a repetitive pattern, which may befor example a fourbit pattern output signal wherein three consecutive lsare followed by one 0 in the NRZ code which is suitable for the systemhaving a f 1, ratio of 5. Signal generator 2t) may be driven by the dataclock of data source 4 so the output signals from signal generator 20will be synchronous with the output signals from data source 4. Thewaveform of the output signal from generator 20 is shown in FIG. 6. InFIG. 5, the output signals from data source 4 and signal generator 29are applied to a logic circuit including inverters 21 and 22, AND gates23 and 24, and OR gate 25. If the negative level of the data patternsfrom data source 4 and signal generator 29 is considered as binary 0 andthe positive level of the signals is considered as binary 1, then thelogic circuit is designed to condition a positive state of the output ofOR circuit 25 when the signals from data source 4 and signal generator29 are both ls or Os concurrently, and a negative state at the output ofOR circuit 25 when the signals from data source 4 and generator 28 are 1and "0 or 0 and 1 concurrently. The positive and negative output signalsfrom OR circuit 25 are added to the data signal from data source 4 atthe summing circuit 26.

The effect of adding the positive and negative output signals of ORcircuit 25 to a typical data signal from source 4 is graphically, shownby reference number 27 in FIG. 7 with the original data signal shown bythe dotted line 28. The voltage amplitude y indicated in FIG. 7 is thevoltage difference between the positive and negative output signals ofOR gate 25 (FIG. 5). The modified data signal as shown in FIG. 7contains harmonies which, when multiplied by the carrier in modulator 7,result in a pair of frequency components in the modulation signal whichare symmetrical about the carfier frequency at a distance less than 7'as illustrated by f equencies f f and f +f in FIG. 1. The frequencies ff and f +f will be passed by VSB filter 9 and will remain in the VSBtransmitted signal as component frequencies which may be utilized toretrieve the carrier at the receiver.

Another embodiment of circuit 5 of FIG. 1 is shown in FIG. 8. In thisembodiment, low frequency components (i.e. a D.C. level) is introducedinto the data signal by a bias distortion. In FIG. 8, circuit 5 includesa singleshot trigger circuit 3t") and a summing circuit 31. Thesingle-shot trigger circuit 3%, connected to the output of data source4, produces a ne ative output pulse in response to the positive-goingvariations of the data signal. The nega tive pulse outputs from triggercircuit 3% are equal in amplitude to the negative to positive amplitudeswing of the data signal. The output pulses from trigger circuit 39 areadded to the data signal from data source 4 at summing circuit 31. Theresult of the summation is graphically shown in FIG. 9. In FIG. 9, thewaveform represented by the solid line 32 is the output signal fromsumming circuit 31 (FIG. 8). The dotted lines 33 represent the positionin time of the positive going variations of the data signal from datasource 4. The time delay At between the positive going variation of theoriginal data signal and the positive going variation of the outputsignal from summing circuit 31 is due to the addition of the negativepulse from trigger 39. For data patterns having no D.C. components thetime delays At occurring at the positive transitions of the data signalresult in a widening of the negative bit period preceding the transitionand a narrowing of the positive bit period following the transitionresulting in a D.C. component in the data pattern. The presence of theD.C. compoent provides a component at carrier frequency in themodulation signal as previously discussed. The width of the negativepulse from trigger circuit 36 can be adjusted in accordance with thenumber of transitions per bit that the data signal undergoes to providean amount of DE. components in the data signal which will not cancel theD.C. components present in those patterns which contain D.C. componentsbut no frequency components less than h. The net D.C. componentintroduced is proportional to the number of transitions per hit that thedata signal undergoes, and also to the width of the negative pulse fromtrigger circuit 30.

Still another way of insuring a pair of frequency components (ie. acarrier component) in the transmitted signal by the addition of a lowfrequency component (D.C.) in the data signal is by the addition ofextra bits to the data signal to maximize the magnitude of the D.C.level. To illustrate, any seven bit character, forexample 1001101, wouldhave a minimum D.C. magnitude of 1/7 units. By the addition of apositive bit the D.C. level is raised to a minimum magnitude of 1/4.Likewise a five bit character such as 0100 would have a minimum D.C.magnitude of 1/5. The addition of a negative bit increases the mini-mumD.C. level to -1/ 3.

The foregoing discussion illustrated various embodiments of circuit(FIG. 2 which may be utilized to provide a pair of frequency componentssymmetrical about the carrier frequency within the range f f to f +f byproviding a low frequency component (including D.C.) in the data signal.It is obvious that other circuitry may be provided for circuit 5 foraccomplishing this desired result, and circuit 5 is intended to broadlyrepresent any means for providing a pair of frequency components asdescribed (which includes a component at the carrier frequency) in thetransmitted signal.

In the embodiments shown, the circuitry for providing such componentpairs in the transmitted signal operated in conjunction with the datasignal from data source 4. It is to be understood that the provision ofthe component pairs in the transmitted signal is not limited to themodification of the data signal, but may be carried out by themodification of signals at other portions of the transmitter, forexample, the introduction of a predetermined amount of carrier signal tothe modulation signal output from modulator 7.

Having indicated how a pair of component frequencies symmetrical aboutthe carrier frequency may be included in the transmitted signal in orderthat the carrier may be retrieved at the receiver for synchronousdemodulation, the circuitry included in the receiver for retrieving thecarrier will now be discussed.

Referring to FIG. 2, receiver 2 includes an amplifier 9, a balanceddemodulator 1t and a low pass filter 11 coupled in series. Coupled tothe output of amplifier 9 is a series arrangement including asymmetrizer circuit 12, a square law detector 13, clipping circuit 14, anarrow band filter 15, a delay network 16, a limiting circuit 17, and adivider circuit 1-8, the output of which is coupled to balanceddemodulator 10.

The specific circuits included within the receiver 2 are conventionalwith the exception of symmetrizer circuit 12; for example, square lawdetector 13 may be a full wave rectifier, clipping circuit 14 may be adiode clipper, narrow band filter 15 may be a tuned circuit tuned totwice the carrier frequency, and delay network 16' may be a phase shiftcircuit and balanced demodulator may be a product modulator similar tocircuit '1' in the transmitter. The function of symmetry circuit 12 isto be responsive to the received VSB signal and to provide a doublesideband signal consisting of the components within the range of f to f-l-f and wherein said components are symmetrical in amplitude and phasewith respect to the carrier frequency. In practice symmetrizer circuit12 may be, for example, a complementary filter having characteristicswhich are the complement of the characteristics of the VSB filter 9. TheVSB filter, as seen in FIG. 1, is ideally a low pas filter having nosignificant output above f +f and with a linear attenuationcharacteristic between f,,- and f d-f The complementary filter employedas symmetri'zer circuit 12 would therefore be a high pass filter with nosignificant output below f -f and a linear attenuation slope between f fand f }-f The characteristic of the complementary filter with respect tothe characteristic of VSB filter 9 is shown in FIG. 10 with Arepresenting the VSB filter characteristic and C representing thecomplementary filter characteristic.

From FIG. 10 it is seen that any VSB signal components below f f will befully "attenuated by the complementary filter. The combined overallcharacteristic of the VSB and complementary filters i illustrated bycurve D. A pair of frequency components within the range f f to f +fsuch as the components f -f and f -l-f shown in FIG. 1 are thus equal inamplitude as shown in FIG. 10. The output signal from the complementaryfilter is, therefore, a double sideband signal symmetrical about thecarrier frequency. Thus, components of the VSB signal which are atcarrier frequency or frequencies within the f if range are applied tofull wave rectifier 13. The square law detector (i.e. full waverectifier) 13 is employed because, in the present example, the carriersignal reverse in phase when the modulating signal changes in polarity.The square law detector 13 removes the phase reversals and produces anoutput signal having a component at 21}. Clipping circuit 14 is includedto reduce the range of amplitude variation of the -2f waveform bychanging the harmonic content and reducing the amplitude which variesover a wide range as a function of the data pattern. The output signalfrom clipping circuit 14 is filtered by narrow band filter 15 which istuned to 2f Delay network 16 is employed to establish the correct phaseof the retrieval carrier relative to the input to demodulator 1t).Limiting circuit 17 is employed to further reduce the range of amplitudevariation of the 2 waveform and also provides a relatively square waveinput signal to dividing circuit 18 which may be a binary trigger vwhichscales the Zf waveform down to an f waveform.

The receiver circuitry described above provides a carrier term havingthe correct frequency and phase for synchronous demodulation in theevent of frequency translation in the transmission medium.

The use of other circuitry is possible Within the scope of theinvention. For example, the chanacteristics desired of symmet-rizercircuit 12 could be roughly approximated by a tuned circuit. Also, thefunction of the narrow band filter 15 of FIG. 2 could be accomplishedusing a phase locked oscillator.

It is to be understood that the present invention is not restricted touse With the bipolar binary NRZ code illustrated in the variouswaveforms, and that this type coding was shown for convenience in thediscussion. Multilevel coding can be employed.

What has beendescribed is a novel system whereby the presence of a pairof frequency components for carrier retrieval are insured in thetransmitted signal by transmitter circuitry. The system further includesreceiver circuitry which retrieves the carrier term from the transmittedsignal.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. In a transmission system, a transmitter for transmit-ting asuppressed carrier vestigial sideband modulation signal comprising incombination:

a source of binary data signal,

a source of carrier signal,

modulating means responsive to said binary data signal and said carriersignal for modulating said carrier signal with said data signal,

a vestigial sideband filter coupled to the output'of said modulatingmeans for filtering the output signal there-from,

and means coupled between said source of binary data signal and saidmodulating means for modifying said data signal with a continuous signalhaving a predetermined amplitude for providing at least one componentfrequency in said transmitted signal approximately at the frequency ofsaid carrier signal and lying within a range of frequencies effectivelypassed by said vestigial sideband filter, said predetermined amplitudeof said continuous signal being derive-d from the maximum pattern lengthof said binary data signal having data patterns with at least onefrequency component greater than the greatest frequency effectivelypassed by said vestigial sideband filter.

2. A transmission system according to claim 1 wherein said means formodifying said data signal with a continuous signal provides a singlefrequency component in said transmitted signal at the frequency of saidcarrier signal.

3. A transmission system according to claim 1 wherein said continuoussignal from said modifying means is a direct current signal having apredetermined amplitude derived from the ratio of the data rate of thebinary data signal to the bandwidth equivalent of carrier recoverybandwidth.

4. A transmission system according to claim 1 wherein said means formodifying said data signals with a continuous signal is a battery and aresistor coupled in series and connected between said source of datasignals and said modulating means.

5. A transmission system according to claim 1 wherein said binary datasignal has a maximum data rate of f wherein said vestigial sidebandfilter provides a baseband equivalent of carrier recovery bandwidth of fand wherein said predetermined amplitude of said continuous signal isderived from data patterns having pattern lengths less than the ratiofd/ f References (Iited by the Examiner UNITED STATES PATENTS Eglin325-331 Rieke 325331 Stachiewicz 32549 Hopner et a1 17867 Beck 17866Myrick 17866 Matsushima 3255O Becker et a1. 32550 15 DAVID G,REDINBAUGH, Primary Examiner.

STEPHEN W. CAPELLI, Examiner.

1. IN A TRANSMISSION SYSTEM, A TRANSMITTER FOR TRANSMITTING A SUPPRESSEDCARRIER VESTIGIIAL SIDEBAND MODULATION SIGNAL COMPRISING IN COMBINATION:A SOURCE OF BINARY DATA SIGNAL, A SOURCE OF CARRIER SIGNAL, MODULATINGMEANS RESPONSIVE TO SAID BINARY DATA SIGNAL AND SAID CARRIER SIGNAL FORMODULATING SAID CARRIER SIGNAL WITH SAID DATA SIGNAL, A VESTIGIALSIDEBAND FILTER COUPLED TO THE OUTPUT OF SAID MODULATING MEANS FORFILTERING THE OUTPUT SIGNAL THEREFROM, AND MEANS COUPLED BETWEEN SAIDSOURCE OF BINARY DATA SIGNAL AND SAID MODULATING MEANS FOR MODIFYINGSAID DATA SIGNAL WITH A CONTINUOUS SIGNAL HAVING A PREDE-